Internet DRAFT - draft-akiya-bfd-intervals
draft-akiya-bfd-intervals
Internet Engineering Task Force N. Akiya
Internet-Draft M. Binderberger
Intended status: Informational Cisco Systems
Expires: May 17, 2014 November 13, 2013
Standardized interval support in BFD
draft-akiya-bfd-intervals-04
Abstract
This document defines a set of interval values that we call "Standard
intervals". Values of this set must be supported for transmitting
BFD control packets and for calculating the detection time in the
receive direction when the value is equal or larger than the fastest,
i.e. lowest, interval a particular BFD implementation supports.
This solves the problem of finding an interval value that both BFD
speakers can support while allowing a simplified implementation as
seen for hardware-based BFD. It does not restrict an implementation
from supporting more intervals in addition to the Standard intervals.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 17, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The problem with few supported intervals . . . . . . . . . . . 3
3. Well-defined, standardized intervals . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Why some intervals are in the standard set . . . . . . 5
Appendix B. Timer adjustment with non-identical interval sets . . 6
Appendix C. Open/upcoming topics . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
The standard [RFC5880] describes how to calculate the transmission
interval and the detection time. It does not make any statement
though how so solve a situation where one BFD speaker cannot support
the calculated value. In practice this may not been a problem as
long as software-implemented timers have been used and as long as the
granularity of such timers was small compared to the interval values
being supported, i.e. as long as the error in the timer interval was
small compared to 25 percent jitter.
In the meantime requests exist for very fast interval values, down to
3.3msec for MPLS-TP. At the same time the requested scale for the
number of BFD sessions in increasing. Both requirements have driven
vendors to use Network Processors (NP), FPGAs or other hardware-based
solutions to offload the periodic packet transmission and the timeout
detection in the receive direction. A potential problem with this
hardware-based BFD is the granularity of the interval timers.
Depending on the implementation only a few intervals may be
supported, which can cause interoperability problems. This document
proposes a set of interval values that must be supported by all
implementations. Details are laid out in the following sections.
2. The problem with few supported intervals
Lets assume vendor "A" supports 10msec, 100msec and 1sec interval
timers in hardware. Vendor "B" supports every value from 20msec
onward, with a granularity of 1msec. For a BFD session "A" tries to
set up the session with 10msec while "B" uses 20msec as the value for
RequiredMinRxInterval and DesiredMinTxInterval. RFC5880 describes
that the negotiated value for Rx and Tx is 20msec. But system "A" is
not able to support this. Multiple ways exist to resolve the dilemma
but none of them is without problems.
a. Realizing that it cannot support 20msec, system "A" sends out a
new BFD packet, advertising the next larger interval of 100msec
with RequiredMinRxInterval and DesiredMinTxInterval. The new
negotiated interval between "A" and "B" then is 100msec, which is
supported by both systems. The problem though is that we moved
from the 10/20msec range to 100msec, which has far deviated from
operator expectations.
b. System "A" could violate RFC5880 and use the 10msec interval for
the Tx direction. In the receive direction it could use an
adjusted multiplier value M' = 2 * M to match the correct
detection time. Now beside the fact that we explicitly violate
RFC5880 there may be the problem that system "B" drops up to 50%
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of the packets; this could be the case when "B" uses an ingress
rate policer to protect itself and the policer would be
programmed with an expectation of 20msec receive intervals.
The example above could be worse when we assume that system "B" can
only support a few timer values itself. Lets assume "B" supports
"20msec", "300msec" and "1sec". If both systems would adjust their
advertised intervals, then the adjustment ends at 1sec. The example
above could even be worse when we assume that system "B" can only
support "50msec", "500msec" and "2sec". Even if both systems walk
their supported intervals, the two systems will never be able to
agree on a interval to run any BFD sessions.
3. Well-defined, standardized intervals
The problem can be reduced by defining interval values that are
supported by all implementations. Then the adjustment mechanism
could find a commonly supported interval without deviating too much
from the original request.
In technical terms the requirement is as follows: a BFD
implementation MUST support all values in the set of Standard
interval values which are equal to or larger than the fastest, i.e.
lowest, interval the particular BFD implementation supports.
The proposed set of Standard interval values is: 3.3msec, 10msec,
20msec, 50msec, 300msec and 1sec.
The adjustment is always towards larger, i.e. slower, interval values
when the initial interval proposed by the peer is not supported.
This document is not adding new requirements with respect to how
exact a timer value must be implemented. Supporting an interval
value means to advertise this value in the DesiredMinTxInterval
and/or RequiredMinRxInterval field of the BFD packets and to provide
timers that are reasonably close. RFC5880 defines safety margins for
the timers by defining a jitter range.
How is the "Standard interval set" used exactly? In the example
above, vendor "A" has a fastest interval of 10msec and thus would be
required to support all intervals in the standard set that are equal
or larger than 10msec, i.e. it would support 10msec, 20msec, 50msec,
300msec, 1sec. Vendor "B" has a fastest interval of 20msec and thus
would need to support 20msec, 50msec, 300msec and 1sec. As long as
this requirement is met for the standard set of values, then both
vendor "A" and "B" are free to support additional values outside of
the standard set.
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4. IANA Considerations
No request to IANA.
5. Security Considerations
This document does not introduce any additional security issues and
the security mechanisms defined in [RFC5880] apply in this document.
6. Acknowledgements
We would like to thank Sylvain Masse and Anca Zamfir for bringing up
the discussion about the Poll sequence. Jeffrey Haas helped finding
the fine line between "exact" and "pedantic".
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
Appendix A. Why some intervals are in the standard set
The list of standard interval values is trying to balance various
objectives. The list should not contain too many values as more
timers may increase the implementation costs. On the other hand less
values produces larger gaps and adjustment jumps. Larger values in
the lower interval range may be easier to support, potentially even
in software instead of hardware.
o 3.3msec: required by MPLS-TP
o 10msec and 20msec: both values allow to detect faster than 50msec,
when used with a multiplier of 2 or 3 (for 10msec). A compromise
could be a single interval of 15msec.
o 50msec: this seems an interval often supported by software
implementations, so the assumption here is that for convenience
this value should be supported.
o 300msec: this would support large scale of 3 x 300msec setups used
by customers to have a detection time slightly below 1sec for VoIP
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setups.
o 1sec: as mentioned in RFC5880. While the interval for Down
packets can be 1sec or larger this draft proposes to use exactly
1sec to avoid interoperability issues.
With that stated, one of the primary intention of this first draft is
to seek feedback on the number of interval values in the standard set
as well as each value.
Candidates for larger intervals to be part of the Standard interval
set would be 10sec, 1min and 10min. Such larger BFD intervals may be
used for BFD graceful restarts.
Appendix B. Timer adjustment with non-identical interval sets
RFC5880 implicitly assumes that a BFD implementation can support any
timer value equal or above the advertised value. When a BFD speaker
starts a poll sequence then the peer must reply with the Final (F)
bit set and adjust the transmit and detection timers accordingly.
With contiguous software-based timers this is a valid assumption.
Even in the case of a small number of supported interval values this
assumption holds when both BFD speakers support exactly the same
interval values.
But what happens when both speakers support intervals that are not
supported by the peer? An example is router "A" supporting the
standard interval set plus 100msec while router "B" support the
standard intervals plus 200msec. Assume both routers are configured
and run at 50msec. Now router A is configured for 100msec. We know
the result must be that both BFD speaker use 300msec timers but how
do they reach this endpoint?
First router A is sending a packet with 100msec. The P bit is set
according to RFC5880. The Tx timer stays at 50msec, the detection
timer is 3 * 100msec:
(A) DesiredTx: 100msec, MinimumRx: 100msec, P-bit
Tx: 50msec , Detect: 3 * 100msec
Router B now must reply with an F bit. The problem is B is
confirming timer values which it cannot support. The only setting to
avoid a session flap would be
(B) DesiredTx: 200msec, MinimumRx: 200msec, F-bit
Tx: 50msec , Detect: 3 * 200msec
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immediately followed by a P-bit packet as the advertised timer values
have been changed:
(B) DesiredTx: 200msec, MinimumRx: 200msec, P-bit
Tx: 50msec , Detect: 3 * 200msec
This is not exactly what RFC5880 states in section 6.8.7 about the
transmission rate. On the other hand as we will see this state does
not last for long. Router A would adjust it's timers based on the
received Final bit
(A) Tx: 100msec , Detect: 3 * 300msec
Router A is not supporting the proposed 200msec and would use 300msec
instead for the detection time. It would then respond to the
received Poll sequence from router B, using 300msec as router A does
not support the Max(100msec, 200msec):
(A) DesiredTx: 300msec, MinimumRx: 300msec, F-bit
Tx: 100msec , Detect: 3 * 300msec
followed by it's own Poll sequence as the advertised timer values
have been changed:
(A) DesiredTx: 300msec, MinimumRx: 300msec, P-bit
Tx: 100msec , Detect: 3 * 300msec
Router B would adjust it's timers based on the received Final
(B) Tx: 200msec , Detect: 3 * 300msec
and would then reply to the Poll sequence from router A:
(B) DesiredTx: 200msec, MinimumRx: 200msec, F-bit
Tx: 300msec , Detect: 3 * 300msec
which finally makes router A adjusting it's timers:
(A) Tx: 300msec , Detect: 3 * 100msec
In other words router A and B go through multiple poll sequences
until they reach a commonly supported interval value. Reaching such
a value is guaranteed by this draft.
Appendix C. Open/upcoming topics
As part of the ongoing BFD workgroup effort the following topics may
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require more discussion and investigation:
Alignment of BFD intervals with Y.1731 intervals. Some contradicting
requirements. Ethernet OAM intervals do not fit intervals of
existing BFD implementations while adding more intervals to the
standards set would complicate hardware implementations.
Larger interval values. To simplify hardware implementation a
question would be if we want to make all the intervals above 1sec
part of the standard interval set.
Jitter. For the very fast interval of 3.3msec, do we want jitter.
Question comes up from hardware teams.
Authors' Addresses
Nobo Akiya
Cisco Systems
Email: nobo@cisco.com
Marc Binderberger
Cisco Systems
Email: mbinderb@cisco.com
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